As a long time Univ. professor, I can assure you this is well within the fair use guidelines. If this was an older child, say 5th grade or up, then including a proper citation is actually an important part of the project.

But this is first grade! Don't fret the citations -- maybe just say where it came from and have fun talking about how the photographer might have taken the shot, and perhaps experiment a bit trying to take your own shot of a backyard 'eagle'.

For a first grader, maybe a lesson could center around writing a short thank you note, a lesson that will always be appropriate later in life. While the citation is appropriate for the older children and up, I think people generally publish online so that others can benefit from the work and they would appreciate hearing about it.

(to be cautious, probably better to route through a parent controlled gmail type address, in case the person turns out to be a bit of an ass.)